901210 - removing flicker from a scrollview

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hi in my application the view is derived from CScrollView. it's usually required to redraw the view after handling user events. for removing flicker, i want to use a memory DC, so that the drawings occur on it and at last i copy it to the client area. i create the memory DC with size of the client. this causes the drawings limit to the size of the client. for example if the whole scroll view is 1000x1000 and the client size is 100x100 by scrolling the view nothing is drawn beyond rectangle (0, 0, 100, 100). i don't want to create a memory DC as big as the whole scroll view. it consumes a large amount of memory most of it is out of the visible part of it.
how can i implement it?
thx

Posted 29-Feb-12 2:50am

ilostmyid2

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Richard MacCutchan · Answer 1 · 2012-02-29T03:24:00

Solution 1

You need to figure out (from the scroll positions) which part of your drawing is to be displayed, and then map that onto the rectangle that is physically at location [0, 0, 100, 100]. For example, if horizontal scroll is +50 and vertical scroll is +100, then you need to draw the logical picture [ 100, 50, 200, 150] onto the DC at [0, 0, 100, 100]. It's just a matter of mathematics.

Posted 29-Feb-12 3:24am

Richard MacCutchan

Comments

ilostmyid2 29-Feb-12 9:38am

when i don't use memory DC and draw directly on screen, i see that just newly exposed parts of the view are invalidated and drawn. i prefer not to change anything but the DC which drawings are done on. i like not to change the ScrollView mechanism and let it do what it thinks that is required on the memory DC instead of client DC. as u see, the client DC size is the same as memory DC which is client size. i tried to override OnPaint and call OnPrepareDC for the memory DC instead of paint DC. after all drawings made on the memory DC, then by using a BitBlt i tried to copy the result onto the client. look at the code below:
void Ct2View::OnPaint()
{
CPaintDC dc(this);
CDC *pDC = d_bAvoidFlicker ? &d_memDC : &dc;
OnPrepareDC(pDC);

OnDraw(pDC);

if (d_bAvoidFlicker)
dc.BitBlt(0, 0, d_clientSize.cx, d_clientSize.cy, pDC, 0, 0, SRCCOPY);
}
but it didn't result to a proper drawing.

Richard MacCutchan 29-Feb-12 10:04am

Yes, but what happens in your OnDraw() function? That is where you have to decide which part of your image must be drawn, based on the position of your two scrollbars.

ilostmyid2 29-Feb-12 10:10am

here's the code:

void Ct2View::pr_getItemLeftTopCorner(CPoint &p, int col, int row)
{
p.x = d_margin.left + col * (CMainFrame::itemSize.cx + d_gap.cx);
p.y = d_margin.top + row * (CMainFrame::itemSize.cy + d_gap.cy);
}

void Ct2View::pr_drawItem(CDC *pDC, int col, int row)
{
CPoint p;
pr_getItemLeftTopCorner(p, col, row);

if (d_pItemsProperties[row][col])
{
d_pItemsProperties[row][col]->drawItemInMemory();
pDC->BitBlt(p.x, p.y, CMainFrame::itemSize.cx, CMainFrame::itemSize.cy, &CMainFrame::tmpDC, 0, 0, SRCCOPY);
}
else
{
CPen pen(PS_DOT, 1, RGB(255,255,255));
CPen *pOldPen = pDC->SelectObject(&pen);
pDC->SelectStockObject(BLACK_BRUSH);
pDC->SetBkColor(BLACK);
CRect r(p, CMainFrame::itemSize);
pDC->Rectangle(r);
CString s;
s.Format(_T("(%d, %d)"), col, row);
CSize sz = pDC->GetTextExtent(s);
pDC->SetTextColor(RGB(255,0,0));
pDC->TextOut(r.left + (CMainFrame::itemSize.cx - sz.cx)/2, r.top + (CMainFrame::itemSize.cy - sz.cy)/2, s);
pDC->SelectObject(pOldPen);
}
}

void Ct2View::pr_drawSelRect(CDC *pDC)
{
CPoint p;
CRect r;

pr_getItemLeftTopCorner(p, d_curCol, d_curRow);
r = CRect(p, CMainFrame::itemSize);
r.InflateRect(5, 5);

pDC->SelectStockObject(NULL_BRUSH); // not to draw inside of rect
pDC->SetBkColor(BLACK); // ta ino nazanim pen-emun dotted nemisheh!
CPen pen(PS_DOT, 1, WHITE);
CPen *pOldPen = pDC->SelectObject(&pen);

pDC->Rectangle(r);

pDC->SelectObject(pOldPen);
}

void Ct2View::OnDraw(CDC* pDC)
{
for (int j=0; j < d_numRows; j++)
for (int i=0; i < d_numCols; i++)
pr_drawItem(pDC, i, j);
pr_drawSelRect(pDC);
}

Richard MacCutchan 29-Feb-12 10:22am

Fine, not that I can be expected to figure out quite what it's doing. But, I ask again, where do you make adjustments for the current scroll position? For example if you wish to draw a line across the client area that is 200 pixels wide and you have scrolled the window right 80 pixels, and the client is 100 pixels wide, then you need to draw from logical point 81 to 180.

ilostmyid2 29-Feb-12 10:38am

i think this is the art of ScrollView that accepts responsibility of locations of things which are drawn inside the scroll view. everything gets ok when i put my code in OnDraw instead of OnPaint. all adjustments get true when it calls OnPrepareDC. then i may suppose that i'm going to draw the whole view which begins from the left top corner of the client area and is wide and high enough to hold the whole content. i may forget scrollbars and forget that the content may be scrolled. as u see in the code, i've not to worry about the scroll position. i just draw and the ScrollView handles the remainder.

ilostmyid2 29-Feb-12 11:41am

hey, i thought more about it and changed OnPaint to:
void Ct2View::OnPaint()
{
CPaintDC dc(this);
CDC *pDC = d_bAvoidFlicker ? &d_memDC : &dc;
OnPrepareDC(pDC);

OnDraw(pDC);

if (d_bAvoidFlicker)
{
CPoint pos = GetScrollPosition();
dc.BitBlt(0, 0, d_clientSize.cx, d_clientSize.cy, pDC, pos.x, pos.y, SRCCOPY);
}
}
now it's working :)
but there's a drawing problem still existing. that's the background is not filled. i tried to put:
pDC->FillSolidRect(0, 0, d_clientSize.cx, d_clientSize.cy, BLACK);
at the beginning of OnDraw. but it didn't help.
why is that, u think?

Richard MacCutchan 29-Feb-12 11:55am

Not sure about that, it may be better to add your own code to capture the WM_ERASEBKGND message and allow the framework to paint it a colour of your choice.

ilostmyid2 1-Mar-12 2:11am

in fact, calling base OnEraseBkgnd causes the client DC's background to be erased, even if the memory DC is passed as the argument, because it implements handling the msg by calling Default().
i think i found the solution. as jkchan said we must draw in the source DC (memory DC) everything required. i agree with 1 and don't agree with 2. it's not our responsibility to take care of the scrollbars positions. in OnDraw we may assume that we're going to draw the whole surface on a large enough bitmap and don't worry about how really large is the DC which are drawing on. if so, my mistake is that i didn't clear the whole surface in OnEraseBkgnd. as i said, i can't call OnEraseBkgnd of base in case i'm drawing on memory DC as source. so i've to use FillSolidRect for clearing the surface before drawing anything on it. this still may occur in OnEraseBkgnd. but the cleared rect must not limit to (0, 0, clientSize.cx, clientSize.cy). i must calculate the whole area of the scrolled thing and fill such a big rect which is the whole surface.
the created bitmap may still be the same size as the client. nothing is drawn out of it. but all clippings are done by the ScrollView itself. it decides when to use ScrollWindow and when to really draw. it tries to draw as less as possible and clips what already exists (drawn before and is not required to be drawn onto it) out of the regions which are drawn.

jk chan · Answer 2 · 2012-02-29T03:26:00

Solution 2

it is usually like this
1. create a source dc which contains the original thing to draw(bitmap,shape or whatever)
2. depends upon your scroll bar position, use BitBlt function to copy the required amount of pixels from source DC to your client dc(whose size is GetClientRect)..

hopes this will help you somewhat.

jkchan
http://cgmath.blogspot.com

Posted 29-Feb-12 3:26am

jk chan

Comments

ilostmyid2 29-Feb-12 9:33am

source DC is the memory DC which size is GetClientRect, so the amount of pixels is not dependent to the scroll position. as i said, i'm not going to make the whole DC.

jk chan 29-Feb-12 23:25pm

you need to create a source dc same size as of the thing to draw.. i don't think creating 1000x1000 size dc will affect the perfomance..